The invention relates to a matrix liquid-crystal display based on the principle of electrically controlled birefringence, having
two plane-parallel outer plates, which, together with a frame, form a cell, PA1 integrated nonlinear elements for switching individual pixels on the outer plates, PA1 a nematic liquid-crystal mixture of negative dielectric anisotropy and high specific resistance which is located in the cell, PA1 electrode layers covered by alignment layers on the insides of the outer plates, and PA1 a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 85 degrees to 95 degrees, PA1 1. MOS (metal oxide semiconductor) transistors on a silicon wafer as substrate, and PA1 2. thin-film transistors (TFT) on a glass plate as substrate. PA1 two plane-parallel outer plates, which, together with a frame, form a cell, PA1 integrated nonlinear elements for switching individual pixels on the outer plates, PA1 a nematic liquid-crystal mixture of negative dielectric anisotropy and high specific resistance which is located in the cell, PA1 electrode layers covered by alignment layers on the insides of the outer plates, and PA1 a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 85 degrees to 95 degrees, PA1 the liquid-crystal mixture has a .DELTA..epsilon. of from about -0.5 to -5, .DELTA..epsilon. being the dielectric anisotropy, and/or in which PA1 the birefringence .DELTA.n of the liquid-crystal mixture is between 0.04 and 0.10, and/or in which PA1 the dielectric constant .epsilon..sub..eta. is greater than or equal to 3. PA1 Q is O or CH.sub.2, PA1 m is 1 or 2, and PA1 n is 0 or 1. PA1 two plane-parallel outer plates, which, together with a frame, form a cell, PA1 integrated nonlinear elements for switching individual pixels on the outer plates, PA1 a nematic liquid-crystal mixture of negative dielectric anisotropy and high specific resistance which is located in the cell, PA1 electrode layers covered by alignment layers on the insides of the outer plates, and PA1 a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 85 degrees to 95 degrees, PA1 R.sup.4 and R.sup.5 are each, independently of one another, an alkyl group, in each case having 1 to 15 carbon atoms, preferably 2 to 12 C atoms, particularly preferably 2-7 C atoms, in which one or more CH.sub.2 groups may be replaced by a group selected from the series comprising --O--, --S--, --CO--, --CH-- halogen--, --CHCN--, --O--CO--, --O--COO--, --CO--O-- and --CH.dbd.CH-- or by a combination of two suitable groups, but in which two heteroatoms are not linked direction to one another, ##STR12## or one of these groups in which one or more CH.sub.2 groups have been replaced by O and/or S or aliphatic and/or aromatic CH groups have been replaced by N, PA1 A.sup.o is, in each case independently, 1,4-cyclohexylene which is unsubstituted or monosubstituted or polysubstituted by halogen atoms, CH.sub.3 groups and/or nitrile groups and in which one or two nonadjacent CH.sub.2 groups may be replaced by --O-- and/or --S-- and/or one --CH--CH.sub.2 -- group may be replaced by --C.dbd.N-- (Cy), or is 1,4-phenylene which is unsubstituted or monosubstituted or polysubstituted by halogen atoms, CH.sub.3 groups and/or nitrile groups and in which one or more CH groups may be replaced by N (Ph), or one of the radicals A.sup.o is alternatively 2,6-naphthylene (Na) or tetrahydro-2,6-naphthylene (4H-Na), optionally substituted by halogen or CN, PA1 Z.sup.o is in each case, independently, --CO--O--, --O--CO--, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 --, --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN-- or a single bond, and PA1 p is 1, 2 or 3. PA1 R.sup.6 and R.sup.7 are each, independently of one another, an alkyl group, in each case having 1 to 15 carbon atoms, preferably 2 to 12 C atoms, particularly preferably 2-7 C atoms, in which one or more CH.sub.2 groups may be replaced by a group selected from the series comprising --O--, --S--, --CO--, --CH--halogen--, --CHCN--, --O--CO--, --O--COO--, --CO--O-- and --CH.dbd.CH-- or alternatively by a combination of two suitable groups, but in which two heteroatoms are not linked directly to one another, PA1 A.sup.1 and A.sup.2 are each, independently of one another, 1,4-cyclohexylene which is unsubstituted or monosubstituted or polysubstituted by halogen atoms, CH.sub.2 groups and/or nitrile groups and in which one or two nonadjacent CH.sub.2 groups may be replaced by --O-- and/or --S-- and/or one --CH--CH.sub.2 -- group may be replaced by --C.dbd.N-- (Cy), or is 1,4-phenylene which is unsubstituted or monosubstituted or polysubstituted by halogen atoms, CH.sub.3 groups and/or nitrile groups and in which one or more CH groups may be replaced by N (Ph), or one of the radicals A.sup.o is alternatively 2,6-naphthylene (Na) or tetrahydro-2,6-naphthylene (4H-Na), optionally substituted by halogen or CN, PA1 A is thiadiazo-2,5-diyl or 2,3-difluoro-1,4-phenylene, PA1 Z.sup.1 and Z.sup.2 are each --CO--O--, --O--CO--, --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, --CH.sub.2 O-- or a single bond, PA1 m is 1 or 2, and PA1 n is 0 or 1, PA1 R.sup.8 and R.sup.9 are each, independently of one another, an alkyl group, in each case having 1 to 15 carbon atoms, preferably 2 to 12 C atoms, particularly preferably 2-7 C atoms, in which one or more CH.sub.2 groups may be replaced by a group selected from the series comprising --O--, --S--, --CO--, --CH--halogen--, --CHCN--, --O--CO--, --O--COO--, --CO--O-- and --CH.dbd.CH-- or alternatively by a combination of two suitable groups, but in which two heteroatoms are not linked directly to one another, PA1 Q.sup.1 and Q.sup.2 are each, independently of one another, 1,4-phenylene which is unsubstituted or monosubstituted or polysubstituted by halogen atoms, CH.sub.3 groups and/or nitrile groups, PA1 A.sup.3 and A.sup.4 are each, independently of one another, trans-1,4-cyclohexylene in which, in addition, one or two nonadjacent CH.sub.2 groups may be replaced by --O-- and/or --S--, or are 1,4-phenylene in which, in addition, one or more CH groups may be replaced by N, PA1 o and p are each, independently of one another, 0 or 1, and PA1 Z.sup.1 and Z.sup.2 are as defined for the formula V PA1 Z.sup.1 is --COO--, --O--CO--, --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, --CH.sub.2 O-- or a single bond, and PA1 A.sup.5 and A.sup.6 are each, independently of one another, trans-1,4-cyclohexylene or unsubstituted or fluorine-substituted 1,4-phenylene. PA1 V (0,0) is the threshold voltage IV] at 0% transmission and a viewing angle of 0.degree. PA1 V (10,0) is the threshold voltage [V] at 10% transmission and a viewing angle of 0.degree. PA1 V (90,0) is the threshold voltage IV] at 90% transmission and a viewing angle of 0.degree. PA1 .DELTA.n is the optical anisotropy measured at 20.degree. C. and 589 nm PA1 .DELTA..epsilon. is the dielectric anisotropy at 20.degree. C. PA1 c.p. is the clearing point [.degree.C] PA1 .eta.(T) is the viscosity [mm.sup.2 s.sup.-1 ] at T.degree.C. PA1 S is the steepness of the characteristic line ##EQU1## .eta. is the specific resistance [.OMEGA..cm]
characterized in that the nematic liquid-crystal mixture contains at least one compound which contains a structural element of the formula 1, 2 or 3 ##STR2##
The principle of electrically controlled birefringence, the ECB effect, or the DAP effect (deformation of aligned phases) were described for the first time in 1971 (M. F. Schieckel and K. Fahrenschon. "Deformation of nematic liquid crystals with vertical orientation in electrical fields", Appl. Phys. Lett. 19 (1971), 3912). This was followed by papers by J. F. Kahn (Appl. Phys. Lett., 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys., 44 (1973), 4869).
The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid-crystalline phases must have high values for the ratio between the elastic constants K.sub.3 /K.sub.1, high values for the optical anisotropy .DELTA.n and values for the dielectric anisotropy .DELTA..epsilon. of between -0.5 and -5 in order to be practicable for high-information display elements based on the ECB effect. Electrooptical display elements based on the ECB effect have a homeotropic edge alignment.
For industrial application of this effect in electrooptical display elements, LC phases which must satisfy a wide range of demands are required. Particularly important here are the chemical resistance to moisture, air and physical influences, such as heat, infra-red, visible and ultra-violet radiation, and direct and alternating electrical fields. Furthermore, industrially useful LC phases are required to have a liquid-crystalline mesophase in a suitable temperature range and low viscosity.
None of the series of compounds having a liquid-crystalline mesophase which have been disclosed hitherto contains a single compound which meets all these requirements. For this reason, mixtures of from two to 25, preferably from three to 18, compounds are generally prepared in order to obtain substances which can be used as LC phases. However, ideal phases cannot easily be prepared in this way since there were hitherto no liquid-crystal materials available which have significantly negative dielectric anisotropy and adequate long-term stability.
Matrix liquid-crystal displays (MLC displays) as defined in the preamble are known. Nonlinear elements which can be used to individually switch the individual pixels are, for example, active elements (i.e. transistors). This is then referred to as an "active matrix", and a differentiation can be made between two types:
In the case of type 1, the electrooptical effect used is usually dynamic scattering or the guest-host effect. The use of monocrystalline silicon as the substrate material limits the display size since even modular assembly of a number of part displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electrooptical effect used is usually the TN effect. A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counter-electrode. Compared with the size of the pixel electrode, the TFT is very small and hardly affects the image at all. This technology can also be extended to fully color-capable image displays, where a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable image element.
The TFT displays disclosed hitherto usually operate as TN cells with crossed polarizers in transmitted light and are illuminated from the back.
The term MLC display here covers any matrix display containing integrated nonlinear elements, i.e. in addition to the active matrix, also displays containing passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays in automobile or aircraft construction. In addition to problems with respect to the angle dependency of the contrast and the response times, problems result in MLC displays due to inadequate specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sept. 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. As the resistance decreases, the contrast of an MLC display worsens. Since the specific resistance of the liquid-crystal mixture generally decreases over the life of an MLC display due to interactions with the internal surfaces of the display, a high (initial) resistance is very important for displays which are required to have acceptable resistance values over a long service life.
The disadvantage of the MLC-TN displays disclosed hitherto is their comparatively low contrast, the relatively high angle dependence and the difficulty of generating grey shades in these displays.
There thus continues to be a great demand for MLC displays having very high specific resistance and at the same time a broad operating temperature range, short response times and low threshold voltage which can be used to generate various grey shades.